1. Field of the Invention
The present invention relates to a memory device.
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, or a light-emitting device each including an oxide semiconductor.
In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.
2. Description of the Related Art
A memory device capable of retaining data that has attracted attention includes a combination of a transistor using silicon (Si) for a semiconductor layer and a transistor using an oxide semiconductor (OS) for a semiconductor layer (hereinafter referred to as an OS transistor) (see Patent Documents 1 and 2).
A recent increase in the amount of data manipulated requires a memory device having high storage capacity. In this situation, Patent Documents 1 and 2 disclose memory devices where multilevel data is stored and read.
Note that in this specification, multilevel data means 2-bit (22=4 levels) or more data.